When some one twitted him on his old age, his reply was, "I too am ready to depart ; but when again I consider that I am in all points in good health and that I can still write and read, I am content to wait." We are told that he wrote down Zeno's lectures on oyster-shells [! cf. LSJ s.v. ὄστρακον II 1] and the blade-bones of oxen through lack of money to buy paper. Such was he; and yet, although Zeno had many other eminent disciples, he was able to succeed him in the headship of the school. (Loeb)
On the assumption, pace the Loeb, that Cleanthes used potsherds, not shucks, it is worth noting that T.C. Skeat does not mention this anecdote about 'ostraca' in his article 'Was papyrus regarded as "cheap" or "expensive" in the Ancient World', Aegyptus 75.1/2 (1995) 75-93.
Cleanthes, it seems, was not based in Egypt. H.C. Youtie remarked in ZPE 67 (1987) 267-281 that papyrus was more expensive outside Egypt, i.e. wherever it was imported. See especially l.c. p. 274. He mentions Cleanthes using ostraca not papyri for economic reasons on p. 271 (without a citation).